The goal of this project is to bring to the verge of clinical practice the promising technique of acoustic radiation force impulse (ARFI) imaging for guiding cardiac ablation therapy. Cardiac ablation is used to treat atrial and ventricular tachyarrhythmias. This therapy is used hundreds of times daily with no direct lesion imaging. The multimodal system we are proposing will enable 1) real time evaluation of lesion growth, 2) visual evaluation of lesion continuity, and 3) guidance and placement of catheters. The successful implementation of this system will shorten procedure times, reduce the incidence of repeat procedures and limit complications due to collateral damage. Complex ablations rely on the creation of transmural and contiguous aggregate lesions to block reentrant conduction. Incomplete lesions result in extended procedure times and frequent repeat procedures. Excess energy delivery leads to significant morbidity causing micro-explosions within the tissue and collateral tissue damage with potentially fatal complications. There is an urgent need for an imaging technology to titrate energy delivery and guide ablation tachyarrhythmia therapy. We will use ARFI imaging to characterize cardiac ablation lesions. ARFI imaging characterizes the spatial distribution of elastic properties that can be used to differentiate ablated from normal myocardium. ARFI imaging will be implemented from an enhanced (128 channel) intracardiac echo (ICE) imaging catheter. The major work proposed here is the implementation of ARFI imaging with the ICE catheter and the integration of this technology into the standard interventional regime of the clinic. Electroanatomical mapping is now the standard tool used to guide catheters in the heart and position lesions at clinically relevant locations. Working in concert with industry, we will develop the technology to integrate ARFI image based lesion evaluation into this standard tool for clinical intervention by exploiting a new ICE catheter equipped with a CARTO(tm) magnetic sensor. This tool will enable registration of the two technologies and allow integration of lesion evaluation into the clinical paradigm. Finally, we will fully characterize the change in elastic properties associated with ablation and demonstrate in an animal model that ARFI image guided therapy can be used to augment existing clinical methods and provide the real time image feedback needed to improve the quality of care. We propose to develop a multimodality imaging system to guide cardiac ablation therapy. The system will exploit catheter based acoustic radiation force impulse imaging to characterize lesion growth. This technology will be integrated into the standard clinical catheter guidance paradigm yielding a complete tool for ablative therapy of cardiac tachyarrhythmias.